This invention relates to improved electronic pulse width modulated control method and means for supplying power to an inductive load with special applications to D.C. motors.
Methods are known for controlling current in an inductive load using a relatively high frequency chopper circuit in the order of 10 KHz for applying power to the load using pulse width modulation (P.W.M.).
P.W.M. controls typically operate in relation to reoccuring P.W.M. periods. A clock or other sequencer typically times the beginning of each P.W.M. period. A power switch (or switches) is typically switched "on" (if not already "on"), applying a pulse of current to the load at the beginning of each P.W.M. period. In the "off" state the power switch removes the power applied to the load. A P.W.M. controller, which may include the power switches, controls the "on" and "off" times of the power switches.
On a pulse-by-pulse basis, each pulse of applied power can be limited in duration by duty cycle control or by current limit control when a predetermined maximum current is flowing either in the load and/or in the power switch which couples the power to the load.
FIG. 10 is a flow chart depicting the "on" and "off" time of the power switch for a known P.W.M. method for driving an inductive load and could be implemented on either a software or firmware controlled computer or in hardware or a combination of the two. The "on" and "off" states of the power switch connect and disconnect a source of power across the inductive load.
One of the significant inductive loads controlled by P.W.M. techniques is that of a motor winding for a D.C. motor.
The flow chart of FIG. 10 contains eight blocks numbered 1-8. The operation within blocks 2, 3, 6 and 8 depicts action items whereas the items in blocks 4, 5 and 7 depict test conditions and branches to be taken depending on the results of the tests. The symbolic notation used within the blocks will become better understood with reference to the description of FIGS. 3-5.
However, the operation can briefly be described as follows. As depicted by FIG. 11, the P.W.M. control is controlled by regular reoccurring clock periods or P.W.M. periods. P.W.M. periods 1, 2, 3 and 4 are shown by way of example. At the beginning of each P.W.M. period, the P.W.M. control forms an "on" or high signal which turns "on" the power switch coupling the source of power across the inductive load. A duty cycle is selected for the "on" period depicted as "ton". When the duty period has elapsed, the P.W.M. control turns the power switch "off", allowing the power to remain "off" for the rest of the P.W.M. period.
The P.W.M. control also monitors the current flowing, either in the inductive load or through the power switch, and when the current reaches a predetermined limit value, the P.W.M. control switches the power switch "off". This occurs during P.W.M. period 3 at 3a.
When the inductive load is a winding or windings of a D.C. motor, there are many constraints that have to be taken into account when selecting a P.W.M. switching frequency. Some of the pertinent constraints are now described.
Switching Losses:
Any power switch used in a chopper circuit has internal switching losses and there may be switching losses in associated circuitry such as a snubber or diode protection device for the power switch. These losses are undesirable from thermal considerations, and are directly proportional to the switching frequency. Therefore, to keep switching losses low it is desirable to keep the switching frequency low.
Avoidance of Current Ramping:
The following two equations govern the rate of rise of current in the motor windings for the "on" and "off" states of the power switch: ##EQU1## where: Vs=D.C. supply voltage
E=Motor back E.M.F. PA1 I=Current flowing in the motor windings PA1 R=Resistance of motor windings PA1 L=Inductance of motor windings PA1 di/dt=Rate of rise of current
In typical power switching applications, current limiting is used to protect the power switches against current rising above the maximum limit allowable for the power switch. Sensing circuitry is used to monitor current in the motor windings and/or the power switch. By sensing the current the power switches can be turned "off" before the current reaches this maximum limit. In practice, a predetermined current limit is set that is known to be inside the switching capabilities of the power switch.
Typically, in D.C. motor applications, a power switch has one side coupled through the motor windings to one rail of a D.C. supply voltage and the other side of the power switch is coupled serially through a sensing resistor to the return of the supply voltage. Also, a free wheel diode may be connected in parallel with the motor windings to allow current decay in the winding when the power switch is "off".
When the power switch is "on," current flows from the supply voltage through the motor windings, the power switch, the current sensing resistor and back through the return. When the power switch is switched "off," however, the current decays through the motor windings and around through the free wheel diode. During the power switch "off" state, current cannot be sensed in the sense resistor. Therefore, even when the maximum allowable level of current is flowing in the motor windings, if the power switch is "off", the current is not sensed. At the start of the next P.W.M. period the power switch is turned "on" if the duty cycle is not zero. Also, depending on the type of power switch, there are constraints which cause a minimum "on" time of the device once it has been switched "on". Such a constraint may be the inherent storage time of a bipolar power transistor.
The implication of this minimum "on" time is that even if a current limit is reached as soon as the power switch is turned "on", the power switch cannot turn "off" until this minimum time has elapsed. Once it has turned "off", current is removed from the sense resistor, and at the start of the next P.W.M. period, this minimum "on" time cycle can be repeated.
For a D.C. motor, when the back E.M.F. (E) is small and the expression E+IR is small with respect to the supply voltage Vs, then, from equations (1) and (2) the rate of rise of current is high relative to its rate of fall.
To ensure current stays at or below the maximum level when current limit control is occurring, it is important that the current falls during the power switch "off" time by at least as much as it rises during the "on" time. If this condition is not met, it can result in failure of the power switch by excessive current build-up occurring over several P.W.M. periods. This mechanism is known as current ramping and is illustrated in the current waveform of FIG. 12.
To control current ramping, it is necessary to have an "off" time of sufficient duration to allow the current to fall by at least as much as it can rise during minimum "on" time. In practice, this requirement then limits the maximum switching frequency or puts constraints on the type of switching device which is suitable.
Another method of controlling current ramping is to continuously sense the current flowing in the motor windings. Thus the current limit sensing can remain active even during the "off" time and be used to prevent the new P.W.M. period from turning "on" the power switch if a current limit is still present. In practice, however, the difficulty and/or cost of continuous current sensing, particularly for multi-phase motor systems, can make this approach restrictive in its application.